Field of the Invention
The present invention relates to a deflector for a microcolumn and, more particularly, to an electrostatic quadrupole deflector.
Description of the Related Art
Currently, microcolumns are applied to devices that use an electron beam, such as electron microscopes, as well as to manufacturing equipments or testers that are used in semiconductor or display industry.
FIG. 1 is a perspective view showing the structure of a micro-electron optical column according to a related art. The micro-electron optical column (also referred to as micro-electron column) includes an electron emission source 110, a source lens 120 including four electrodes 121, 122, 123, and 124, a deflector 130, and a focusing lens 140 including electrodes 141, 142, and 143. The electrodes have respective electron passage holes 121a, 122a, 123a, 124a, 141a, 142a, and 143a. The source lens 120 and the focusing lens (or einzel lens) 140 each include two or more electrodes as needed. The deflector system 130 that is driven by an electrostatic field and is composed of two rows of deflectors, an upper deflector 131 and a lower deflector 132. This deflector typically has an octupole structure composed of eight electrodes. The deflector system 130 may also be composed of one or more deflectors 131 and 132 as necessary. FIG. 1 shows an example in which an octupole deflector is used. An electron beam emitted by the microcolumn is scanned to a target T.
FIGS. 2 to 4 show examples of the structures of microcolumns in which an electrostatic octupole deflector according to a related art is used.
FIGS. 5A and 5B are a plan view and a side view that illustrate an octupole deflector according to a related art. With reference to FIGS. 5A and 5B, the fabrication process of a micro electro-mechanical system (MEMS) for an octupole deflector will be described in brief. First, a silicon wafer is dipped in KOH solution so that the silicon wafer is thinned to have a thickness of several hundred micrometers. Next, electrode patterns are formed on the silicon wafer through optical lithography. Next, deep reactive ion etching is performed on the silicon wafer to form eight symmetrical electrodes 1-1′, 1-2′, 1-3′, 1-4′, 1-5′, 1-6′, 1-7′, and 1-8′ that are identical in size, arc-shaped, and are weakly connected to a portion that is to become an electron beam passage hole. An arc angle θ of each electrode of the deflector is 45°. Next, the silicon wafer is fixed to a Pyrex substrate in which an electron beam passage hole “a” is formed through an anodic bonding process. Next, unnecessary portions that are connected to the eight electrodes are removed, leaving an octupole deflector that is structurally stable. The octupole deflector may also be formed by an alternative fabrication process. That is, a silicon wafer, which is thinned to a thickness of several hundred micrometers, is first fixed to a Pyrex substrate in which an electron beam passage hole is formed through anodic bonding; electrode patterns are subsequently formed on the silicon wafer through optical lithography; and the silicon wafer is finally etched by deep reactive etching. According to the latter fabrication process, the octupole deflector can be fabricated more easily.
In the microcolumn according to the related art shown in FIG. 1, the deflector is located at a preceding stage to the einzel lens that serves as the focusing lens. Therefore, in order to increase the size of effective deflection field by causing an electron beams to pass through a center portion of the einzel lens when an electron beam is deflected, it is necessary to use a double octupole deflector system composed of deflectors 131 and 132. In this case, a predetermined deflection voltage is applied to the electrodes 1-1′ to 1-8′ of the octupole deflector shown in FIGS. 5A and 5B. Accordingly, in the case of the octupole structure, a complex circuitry algorithm is needed to drive a deflector system.
Alternatively, in the case of the deflectors 131 and 132 shown in FIG. 2, the deflector 131 shown in FIG. 3, and the deflector 130 shown in FIG. 4, only one octupole deflector may be used for deflection of an electron beam. In this case, in the structure of the deflector shown in FIG. 3, a deflection voltage is applied to only upper, lower, left, and right electrodes 1-1′, 1-3′, 1-5′, and 1-7′ of the octupole deflector and the other electrodes 1-2′, 1-4′, 1-6′, and 1-8′ are grounded.
In the case of the deflector 131 shown in FIG. 3 and the deflector 130 shown in FIG. 4, a conventional quadrupole deflector having a simple driving system may be used instead of an octupole deflector. In this case, although the number of electrodes is four, that is, the structure of the quadrupole deflector is simpler compared to an octupole deflector, when the quadrupole deflector is fabricated in the same method as an octupole deflector, the size of gaps between electrodes is larger in the quadrupole deflector, thus handling deformation easily occurs during the following process: forming of the symmetric identical four electrodes to be weakly connected to a portion in which an electron beam passage hole is to be formed later; fixing of the electrodes to a Pyrex substrate in which the electron beam passage holes is formed, through anodic bonding.
The term “effective deflection field size” means the size of a deflection field that ranges from the center of a target to a position at which the spot size of a deflected electron beam is 20% larger than the spot size of a non-deflected electron beam at the center of the target. In typical electron optical deflection systems, the spot size of an electron beam increases at a sharp rate as an electron beam spot becomes closer to the periphery of a target due to a difference in geometric focal distance that changes from the center to the periphery of the target and distortion of an electron beam that is attributable to spherical aberration of an electron lens that is caused by electric field. The increase in the spot size results in deterioration in resolution, causing a decrease in effective deflection field size. Therefore, it is urgent to minimize the distortion of a deflected electron beam.